Process of burning solid fuel to produce slag suitable for portland cement



United States Patent PROCESS OF BURNING SOLID FUEL TO PRODUCE SLAG SUITABLE FOR PORTLAND CEMENT Thomas H. Oster, Dearborn, Mick, assignor to Ford Motor Company, Dearborn, Mich, a corporation of Delaware No Drawing. Application November 19', 1952, Serial No. 321,501

5 Claims. (Cl. 263-53) This invention is concerned with the art of the. combustion of fuel and more particularly the combustion of fuel as related to the production of Portland cement. This invention is especially well adapted'to the utilization of high ash fuels, although where economically feasible may also be employed with low ash fuels, or the liquid or gaseous fuels.

As the supply of high grade fossil fuels are becoming somewhat depleted it has become imperative from an economical standpoint to find suitable substitutes. Many of the more readily available fossil fuels are characterized by a high ash content and by the fact. that this ashcontent exhibits a low fusion point. This combination of circumstances renders the combustion of these fuels difficult and inefhcient in the ordinary type of coal burner due to the erosion of the brick work by molten slag and by the inevitable partial'insulation of the heat absorbing surfaces of the boiler by the ash. In an eifort' to overcome these difliculties the arth'as developed a fossil fuel burning device termed thecyclone burner.

Briefly this device comprises a water cooled'steel cylinder with its axis slightly inclined from the horizontal and provided at one end'with means for the reception of a stream of crushed fuel and primary air and at'the other endwith an exit for the products of combustion. This type of burner and its operating characteristics areamply and ably described in an article entitled The horizontal cyclone burner by A. E. Grunert et al., appearing at page 613 et seq. of the American Society of 'Mechani'cal Engineers, Transactions, volume 69, 1947.

In the conventional operation of aburner of this type a stream of coal, crushed to pass a inch screen is permitted to flow into the inlet end of the burner with a small stream of primary air which may or maynot be preheated. As described by Grunert et al. the bulk of air needed for combustion is injected through a tangential opening under a pressure of approximately. 20 to 40 inches of water. This pressure combined with the tan gential entrance of the secondary air stream is sufficient to cause the gases within the burnerto describe a helical path in its travel from the inlet to the outlet of the burner. The interior of the burner is maintained at a temperature sufficiently high to melt or gasify all of the products of combustion with theexception of carbon. All of .the ash of the coal is of course melted and hurledagainst'the wall of the burner by centrifugal force. Byvirtue 'ofiits sticky nature, this molten ash entraps particles'of coal and coke and exposes .them to thescouring actioniofthe burner gas until combustion is complete.

This invention contemplates themodification' ofthe conventionalfoperation of the cyclone burner by adding to the burner a stream of crushed, but not necessarily pulverized minerals which arecapable of combining with each other and with the ash of the fuel, to produce a slag capable of grinding into a commercially acceptable'Portland cement. Basically Portland cementis a mixture of dicalciumsilicate, tricalciumsilicate and tricalcium aluminate, usually modified by the presence of small 2,745,657 Patented May 15, 1956 tions prevailing at any given time and location. The coal ash will of course contribute to the formation of the cement composition.

The amount of cement making materials to be added to the burner is adjusted in View of other conditions to give a burner temperature not lower than the which will produce a su'fii'ciently fluid slag to be handled in the burner. Too'low a temperature will result in an incompletely reacted cement product and an unduly heavy build up on the walls of the burner. Too high a temperature will result in too thin a layer of solidified slag on the interior of the burner and too high a temperature of exit gas, both of which tend to increase the consumption of fuel. The liquid slag travels a spiral course around the inner periphery of the burner and is tapped out at the lower end of the burner which is usually the end at which the burner gas exits. This molten slag need only be ground to the proper degree of fineness and admixed with the desired amount of gypsum to yield a highly satisfactory cement.

The gas leaving the cyclone burner will be highly heated since it must be well above the point of fusion of the cement slag. The temperature of this gas will usually be at least 2600 F. and inay be much higher. In order to effect economy of fuel, each cyclone burner is located immedately adjacent at least two regeherators. These regenerators comprise a vertical steel shell located adjacent the cyclone burner and arranged to be connected alternately to the exit of the cyclone burner. The inten'oro'f these r'egenerat'or's are protected by either a layer of refractory, or by internal or external water cooling or both. Adv'antageously' the water cooling of the shell of these regenerators may be similar to that employed in the burner, that is a coil of pipe adjacent the interior wall and designed to conductc'ooling water therethrough. The opening in each regenerator which is designed to be connectable to the outlet of the burner is also connectable' to the airinlet's of the cyclone burner for a purpose which will become obvious as the description proceeds;

The regenerators are arranged to receive a charge of cement making material at the top and to discharge this material at a convenient point. The inlet for the cement making material is arranged so that it can be closed in a substantially airtight manner and a current of air under sui ficient pressure to operate the cyclone burner introduced therethrough'.

Atjth'e start of the cement making process both regenerators have been charged with a sufiiciently refractory cement making material which has been crushed to the proper size but not pulverized; In fact all fine powder should be eliminated from this charge. The outlet of the'first'regenerator is' connected to the outlet of the cyclone'hurner'and'its'charging opening is left open. The outlet of the second recuperator or regenerator is conne'cted'to the air'inlets of the cyclone, and the fan which supplies airto the cyclone is connected to the charging opening 'of the second regenerator. When the burner is first'lighted, all of theh'eat'escaping the burner exit'is employed in raising the contents of the first regenerator to a high temperature. The charge of this recuperator or regenerator will usually be'crushed limestone, or oyster shells'or some similar calcic'material which is extremely refractory. innature; The heat from the cyclone will first dry,then heat," and'then decompose the calcic material to'yielda'highly heatedc'rude' calcium' oxide which will unite "readily and strongly 'exothermi'cally with silica to form the dicalcium silicate and tricalcium silicate which are the basis of Portland cement. When the contents of the first regenerator have become suficiently heated and decomposed, a portion of the material so obtained is re moved, mixed with the proper quantity of other cement making materials and fed into the cyclone burner along with or separately from the coal feed. It is preferred that the very hot calcium oxide leaving the first regenerator be mixed immediately with'the other cement making ingredient to conserve heat and give a mix cool enough to handle. The air and gas connections are then reversed so that the output of the cyclone enters the bottom of the second regenerator, the fan is connected to the top of the first regenerator and the air inlet of the cyclone is connected to the bottom of this first regenerator. Sufficient lime or other cement making material is added to the first regenerator to replenish that withdrawn for charging into the cyclone. This connection causes the air feed for the cyclone to pass over the highly heated calcic material in the first recuperator or regener ator and to become very hot prior to its entry into the cyclone. It is apparent that as the calcic material descends through the recuperator or regenerator, it will become hotter and hotter and finally reach the temperature at which it decomposes into calcium oxide. The size of the calcic material should be chosen so that ample time is permitted at the high temperature for the interior of the biggest lumps to become thoroughly decomposed into calcium oxide. In most installations this will result in gas leaving the regenerators at too high a temperature to discard. This gas should be passed through a waste eat boiler, preferably operating in conjunction with the water passing through the cooling coils of the cyclone burner and regenerator or recuperators. This gas may well be employed to dry thoroughly the coal and more particularly the cement materials, not treated in the regenerator. It is apparent that any moisture present in the untreated cement making materials will hydrate an equivalent amount of calcium oxide. The subsequent decomposition of the calcium hydroxide so produced will require the use of high level heat in the burner which can ill be afforded.

The process as above described is adapted for locations where it is not desired to produce substantial amounts of steam and/or electrical power in conjunction with cement. Where there is a need of or a ready market for steam or electrical energy, some or all of the heat recovery apparatus may be dispensed with and an ordinary boiler substituted. This boiler should operate in conjunction with the cooling water circulating through the cyclone burner. Under these circumstances only sufficient cement making material is supplied to the burner dong with the coal to reduce the burner temperature to a point where the slag produced will remain sufiiciently fluid for tapping out. The heat resident in the gas discharged from the cyclone under these circumstances will be recovered in a boiler located adjacent the cyclone as in conventional power house practice.

Under some circumstances it may be desirable to substitute liquid or gaseous fuel for a portion of the coal. This is particularly true where the coal ash is unduly high in iron compounds and it is necessary to add fuel to the system without the iron compounds. This is due to the fact that most Portland cement specifications limit the amount of iron which may be present. It is of course possible to operate such a burner exclusively on gas or liquid fuel or a mixture thereof, but such will ordinarily not be found to be economical.

While all of the cement making 'materials may be added in the primary air stream along with the coal it usually will be found to be advantageous to add at least a portion of the cement making materials with the secondary air. The addition of large amounts of quite refractory. cement making materials directly along withthe coal will result in an undesired build up of unmelted material in the zone in which the combustion of coal has been initiated and prior to a temperature above the fusion point of the cement making materials having been attained. When the bulk of the cement making materials are added along with the secondary air, the ash of the coal fuses on the walls of the cyclone burner and provides a liquid medium for promoting action between the highly refractory components of the mixture and the siliceous components to produce the dicalcium silicate and tricalcium silicate which exhibit a melting point sufficiently low to be handled conveniently in the burner. The addition of these refractory materials with the secondary air also imparts to them a tangential movement which results in more uniform mixture being produced within the burner. When a particularly abrasive cement making material is employed it may be desirable to split the stream of secondary air into two components, one being directed tangentially of the burner as usual, and one directed radially and to add the cement making materials to the radial stream to avoid the mechanical abrasion of the walls of the burner by the cement making material before the particles thereof have been softened by heat.

In the event that the coal employed is unusually low in ash or has an ash whose melting point is unusually high it may be necessary to recirculate a portion of the slag produced by the burner into one of the entering air streams to produce immediately a slag having a suitable melting point. This may be the raw hot slag as produced or it may be the slag at any stage of subdivision down to and including the finished cement preferably prior to the addition of gypsum or other set controlling chemicals.

I claim as my invention:

l. The process of burning solid fuel and producing a slag suitable for grinding into Portland cement comprising injecting a mixture of air, crushed solid fuel, calcic material, siliceous material and argillaceous material into a water cooled metallic cylinder, burning said solid fuel in the water cooled cylinder andtwith a second stream of air introduced tangentially into said metallic cylinder, said tangentially introduced second stream of air producing a helical movement of the liquid and solid materials along the inner periphery of the shell, maintaining the temperature within the cylinder sufficiently high to melt and gasify respectively all the reactants except carbon, permitting the coal ash and cement making materials to interact, and tapping oil a molten slag.

2. The process of burning solid fuel and producing a slag suitable for grinding into lortland cement comprising injecting initially into the cyclone burner a mixture of solid fuel and primary air causing the combustion of the coal with said primary air and with secondary air injected tangentially, injecting intothe burner along with the secondary air a mixture of calcic material, siliceous material and argillaceous material, said injection taking place at a location in the burner where a layer of molten slag has coated the interior of the burner, said tangentially introduced secondary stream of air producing a helical movement of liquid and solid material along the inner pheriphery of the burner, maintaining temperature within the burner sufficiently high to melt and gasify respectively all the reactants except carbon, permitting the coal ash and cement making materials to interact, and tapping off a molten slag.

3. The process of burning solid fuel and producing a slag suitable for grinding into Portland cement comprising injecting initially into the cyclone burner a mixture of solid fuel and primary air causing the combustion of the coal with said primary air and with secondary air, injecting into the burner along with the secondary air in a tangential direction a mixture of calcic material, siliceous material and argillaceous material, said injection taking place at a location in the burner where a layer of molten slag has coated the interior of the burner and producing a helical movement of liquid and solid material along the inner periphery of the burner, maintaining temperature within the burner suficiently high to melt and gasify respectively all the reactants except carbon, permitting the coal ash and cement making materials to interact, and tapping off a molten slag.

4. The process of burning solid fuel and producing a slag suitable for grinding into Portland cement comprising injecting initially into the cyclone burner a mixture of solid fuel and primary air, causing the combustion of the coal with said primary air, and injecting with secondary air in a radial direction a mixture of calcic material, siliceous material and argillaceous material, said injection taking place at a location in the burner where a layer of molten slag has coated the interior of the burner, maintaining the temperature Within the burner sufficiently high to melt and gasify respectively all the reactants except carbon, permitting the coal ash and cement making materials to interact, and tapping off a molten slag.

5. The process of burning solid fuel and producing a slag suitable for grinding into Portland cement comprising injecting initially into the cyclone burner a mixture of solid fuel and primary air, causing the combustion of the coal with said primary air and with secondary air, injecting into the burner along with the secondary air a mixture of calcic material, siliceous material and argillaceous material and the product obtained by fusing these materials, said injection taking place at a location in the burner where a layer of molten slag has coated the interior of the burner, maintaining temperature within the burner sufficiently high to melt and gasify respectively all the reactants except carbon, permitting the coal ash and cement making materials to interact, and tapping oil a molten slag.

References Cited in the file of this patent UNITED STATES PATENTS 71,776 Maunton Dec. 3, 1867 166,977 Eustis Aug. 24, 1875 685,064 Schubert Oct. 22, 1901 1,893,913 Saint-Jacques Jan. 10, 1933 2,357,301 Bailey et a1. Sept. 5, 1944 FOREIGN PATENTS 2,096 Great Britain 1853 1,467 Great Britain 1865 

1. THE PROCESS OF BURNING SOLID FUEL AND PRODUCING A SLAG SUITABLE FOR GRINDING INTO PORTLAND CEMENT COMPRISING INJECTING A MIXTURE OF AIR, CRUSHED SOLID FUEL, CALCIC MATERIAL, SILICEOUS MATERIAL AND ARGULLACEOUS MATERIAL INTO A WATER COOLED METALLIC CYLINDER, BURNING SAID SOLID FUEL IN THE WATER COOLED CYLINDER AND WITH A SECOND STREAM OF AIR INTRODUCED TANGENTIALLY INTO SAID METALLIC CYLINDER, SAID TANGENTIALLY INTRODUCED SECOND STREAM OF AIR PRODUCING A HELICAL MOVEMENT OF THE LIQUID AND SOLID MATERIALS ALONG THE INNER PERIPHERY OF THE SHELL, MAINTAINING THE TEMPERATURE WITHIN THE CYLINDER SUFFICIENTLY HIGH TO MELT AND GASIFY RESPECTIVELY ALL THE REACTANTS EXCEPT CARBON, PERMITTING THE COAL ASH AND CEMENT MAKING MATERIALS TO INTERACT, AND TAPPING OFF A MOLTEN SLAG. 